1. Field of the Invention
This invention relates to a surface acoustic wave device such as a surface acoustic wave filter or a surface acoustic wave resonator and a communication device equipped with the surface acoustic wave device used in a mobile communication device such as a mobile phone or the like.
2. Description of the Related Art
The surface acoustic wave filter has been widely used as a frequency selection filter used in an RF (Radio Frequency) stage in the mobile communication devices such as a mobile phone and an automobile telephone. In order to reduce a size, a weight and a cost of the mobile communication device, the number of components used in the mobile communication device has been sought to be reduced, and the surface acoustic wave filter has been required to include additional functions. For example, there is a requirement for an unbalanced input-balanced output type surface acoustic wave filter or a balanced input-unbalanced output type surface acoustic wave filter (hereafter referred to as a balanced type surface acoustic wave filter).
In order to satisfy the requirement, an in-line coupled double mode filter is used often. When used as an RF filter, it is often required that one of connecting terminals is of an unbalanced connection with an input/output impedance of 50Ω and the other connecting terminal is of balanced connection with an aligned input/output impedance of 100-200 Ω.
The unbalanced input-balanced output type surface acoustic wave filter means a surface acoustic wave filter having an unbalanced input and a balanced output. The balanced input-unbalanced output type surface acoustic wave filter means a surface acoustic wave filter having a balanced input and an unbalance output. The balanced input or the balanced output means that a signal is inputted or outputted as a difference in electric potential between two signal lines carrying signals that are equal in amplitude and reverse in phase to each other. On the other hand, the unbalanced input or the unbalanced output means that a signal is inputted or outputted as an electric potential on a single line against a ground electric potential.
FIG. 12 is a plan view schematically showing a conventional balanced type surface acoustic wave filter. The balanced type surface acoustic wave filter has surface acoustic wave filters 212 and 213 disposed on a piezoelectric substrate 201 and connected in parallel to each other. The surface acoustic wave filter 212 has three IDT (Interdigital Transducer) electrodes 202, 203 and 204 and reflector electrodes 208 and 209 disposed on both sides of the IDT electrodes 202, 203 and 204. Also, the surface acoustic wave filter 213 has three IDT electrodes 205, 206 and 207 and reflector electrodes 210 and 211 disposed on both sides of the IDT electrodes 205, 206 and 207.
The surface acoustic wave filters 212 and 213 are connected in parallel to each other and together connected to an unbalanced signal terminal 214. The IDT electrodes 202, 204, 205 and 207, which are connected together to the unbalanced signal terminal 214, generate surface acoustic waves when an electric field is applied across a pair of comb-shaped electrodes facing each other in each of them. The generated surface acoustic waves are propagated to the IDT electrodes 203 and 206, each located in the middle of each of the surface acoustic wave filters 212 and 213, respectively. A phase of the IDT electrode 203 in the middle of the surface acoustic wave filter 212 is reverse to, i.e. 180 degrees different from, a phase of the IDT electrode 206 in the middle of the surface acoustic wave filter 213. The surface acoustic waves are propagated from each of the middle IDT electrodes 203 and 206 to each of balanced output signal terminals 215 and 216, respectively, from which the surface acoustic waves are outputted as balanced outputs. Here, the phase of the IDT electrode 203 means a phase of an electric signal that is outputted from the IDT electrode 203. Similar descriptions are used hereafter for the sake of simplicity. A balanced-unbalanced conversion function is realized with the structure described above.
An unbalanced-balanced conversion function can be realized by using the conventional balanced type surface acoustic wave filter shown in FIG. 12. However, the conventional balanced type surface acoustic wave filter is poor in balance. Here, the balance includes amplitude balance and phase balance. When a signal is inputted or outputted as an electric potential difference between two signal lines, the amplitude balance is as better as closer to each other absolute values of amplitudes of signals on the signal lines are. Also, the phase balance is as better as closer to 180 degrees a difference between phases of the signals is.
On the other hand, a balanced type surface acoustic wave element shown in FIG. 13 is proposed. An example of this structure is disclosed in Japanese Patent Application Publication No. H11-97966. An unbalanced terminal 221 is connected with an IDT electrode 203 in the middle of a first stage of an in-line coupled double mode surface acoustic wave filter, which includes three IDT electrodes 202, 203 and 204 interposed between reflector electrodes 210 and 211. Each of the IDT electrodes 202 and 204 in the first stage is serially connected with each of IDT electrodes 205 and 207 in a second stage, respectively. An IDT electrode 206 in the middle of the second stage is divided into two portions which are configured to be reverse in phase to each other and connected to balanced signal terminals 222 and 223, respectively. With this, an unbalanced input with an input impedance of 50Ω and a balanced output with an output impedance of 200Ω are obtained.
It is also possible to use a structure in which the two stage surface acoustic wave element shown in FIG. 13 is serially connected with another in-line coupled surface acoustic wave element that has three IDT electrodes closely disposed along a direction of propagation of the surface acoustic waves and reflector electrodes on both sides of them. Even with this structure, however, the amplitude balance and the phase balance are not good enough.
In Japanese Patent Application Publication No. 2002-84164, there is proposed a structure to improve the balance of conventional double mode surface acoustic wave resonator filter by forming even-number of pairs of IDT electrodes that are disposed in the middle of three IDT electrodes aligned in the propagation direction of surface acoustic waves. However, the amplitude balance and the phase balance of a balanced type surface acoustic wave filter of the structure mentioned above are not good because a polarity of outermost finger of the middle IDT electrode and a polarity of outermost finger of the IDT electrode adjacent the middle IDT electrode differ between left and right and thus parasitic capacitances formed at balanced signal terminals differ from each other.
Therefore, improving the amplitude balance and the phase balance has been sought in the conventional balanced type surface acoustic wave filters. In addition, communication devices using the conventional balanced type surface acoustic wave devices have been not satisfactory in sensitivity.